Research Paper
Volume : 4 | Issue : 12 | December 2015 • ISSN No 2277 - 8179
Modelling of Statcom Utilising Cascaded 15
Level Inverter (H Bridge)
RAKESHKUMAR B.
SHAH
Engineering
KEYWORDS : Static synchronous compensator, Cascaded multilevel inverter
(H-bridge), ABC coordinates
Lecturer, Electrical Engineering Department, R. C. Technical Institute, Sola, Ahmedabad.
(GUJARAT)
ABSTRACT
Among flexible AC transmission system (FACTS) controllers, the STATCOM have shown feasibility in terms of cost
effectiveness in a wide range of problem-solving abilities from transmission to distribution levels. A cascade multilevel inverter is a power electronic device built to synthesize a desired AC voltage from several levels of DC voltages. A method is presented
showing that a cascade multilevel inverter can be implemented using only a single DC power source and capacitors. To control the reactive
power instantaneously, this system is modelled using the ABC transform which calculates the instantaneous reactive power. The simulation result of MATLAB/Simulink software indicates the superior performance of the proposed control system, as well as the precision of the
proposed models.
INTRODUCTION
A cascade multilevel inverter can be implemented using
only a single DC power source and capacitors.Among flexible AC transmission system (FACTS) controllers, the STATCOM have shown feasibility in terms of cost effectiveness
in a wide range of problem-solving abilities from transmission to distribution levels. A cascade multilevel inverter
is a power electronic device built to synthesize a desired
AC voltage from several levels of DC voltages. To operate
a cascade multilevel inverter using a single DC source, it
is proposed to use capacitors as the DC sources. A standard cascade multilevel inverter requires ‘s’ DC sources for
2s+1 level. To be able to operate in a high-voltage application, a large number of DC capacitors are utilized in a
cascaded multilevel inverter-based STATCOM. To obtain a
low distortion output voltage or a nearly sinusoidal output
waveform, a triggering signal should be generated to control the switching frequency of each power semiconductor
switch.
OBJECTIVE
The objective is to come out with a simulation model of
STATCOM based cascaded fifteen inverter and analyzes its
operation.
COMPARATIVE ANALYSIS OF
INVERTERS
CASCADED MULTILEVEL
Multilevel inverters include an array of power semiconductors and capacitor voltage sources, the output of which generate voltages with stepped waveforms. By increasing the
number of levels in the inverter, the output voltages have
more steps generating a staircase waveform, which has a
reduced harmonic distortion. However, a high number of
levels increases the control complexity and introduces voltage imbalance problems. For example, 11-level cascaded
H-bridge inverters will have 5 SDCSs and 5 full bridges.
Similarly, 13-level cascaded H-bridge inverters will have 6
SDCSs and 6 full bridges. And, 15-level cascaded H-bridge
inverters will have 7 SDCSs and 7 full bridges.
Simulation Circuit
It is well known that the amount and type (capacitive or
inductive) of reactive power exchange between the STATCOM and the system can be adjusted by controlling the
magnitude of STATCOM output voltage with respect to
that of system voltage. Figure 1 shows simulated circuit of
STATCOM with 15-level cascaded multilevel inverter.
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Figure 1 Simulated circuit of the STATCOM with 15-level
cascaded multilevel inverter
The reactive power supplied by the STATCOM is given by equation (1),
Where, Vstatcom and Vs are the magnitudes of STATCOM output
voltage and system voltage respectively and X is the equivalent
reactance between STATCOM and the system. When Q is positive, the STATCOM supplies reactive power to the system. Otherwise, the STATCOM absorbs reactive power from the system.
The phase output voltage is synthesized by the sum of individual
inverter outputs,
i.e. Vca= Vca1+Vca2+Vca3+…………. +Vcan
Therefore, the phase voltage for 15-level cascaded inverter is,
Vca=Vca1+Vca2+Vca3+Vca4+Vca5+Vca6+Vca7
Control of DC Capacitor Voltage
If all the components were ideal and the STATCOM output voltage were exactly in phase with the system voltage, there would
have been no real power exchange between STATCOM and system therefore the voltages across the DC capacitors would have
been able to sustain.
However, a slight phase difference between the system voltage
and the STATCOM output voltage is always needed to supply a
small amount of real power to the STATCOM to compensate the
component loss so that the DC capacitor voltages can be maintained. This slight phase difference is achieved by adjusting the
phase angle of the sinusoidal modulating signal. If the real power delivered to the STATCOM is more than its total component
loss, the DC capacitor voltage will rise, and vice-versa. This real
Research Paper
power exchange between STATCOM and the system is described
by Equation (2) below,
(2) Where,
is the phase angle difference between STATCOM
voltage and the system voltage.
SIMULATION RESULT
Figure 2 shows the cascaded fifteen level inverter output phase
voltage waveform. It can be seen that the sinusoidal waveform
and the fifteen level cascaded inverter output waveform both are
approximately same. But the square wave output has a high harmonic content, not suitable for certain AC loads such as motors
or transformers.
Volume : 4 | Issue : 12 | December 2015 • ISSN No 2277 - 8179
CONCLUSION AND FUTURE SCOPE
Conclusion
To achieve as stable a system as possible, a well-defined model
and an effective DC-link-balancing method are necessary. The
cascaded seven-level, nine-level, eleven-level, thirteen-level and
fifteen-level inverters have been used as the studied system.
And, the cascaded fifteen level inverter based STATCOM is designed according to the simplified model in abc coordinates. The
simulation results show superior performances of the designed
abc coordinates inverter based STATCOM. The STATCOM has
the great advantage of a fewer number of devices. The VSI is extremely fast in response to reactive power change. The simulation of the STATCOM is performed in the Simulink environment
and the results are presented.
Future Scope
• A multi-level Stat-Com/BESS is more versatile and flexible
than a conventional Stat-Com because it can
• Control both active and reactive power simultaneously and
independently
• Charge batteries by absorbing active power from the grid
• Be rated higher because of multilevel topology
• Be effective in power oscillation damping
Figure 2 Approximately Sinusoidal Fifteen level inverter output phase voltages
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